I know how to setup the ethernet shield on my local network. What do I need to be able to share a page with the world? (Like you could access it from your house)I know I need a domain name and the shield, but do I need to call my ISP or anything?

Legally, you're probably required to, and you're probably also not allowed to set up the page, anyway: your "home service" contract probably has a "no servers allowed" clause. Your ISP doesn't want you sucking up the entire neighborhood's upload bandwidth serving up porn for profit. Especially not if they're also in the business of selling high-priced "commercial" connections.

As a practical matter, they're almost certainly not going to care if your Arduino is serving up a 200-byte page of temperature readings a few dozen times per day. Almost certainly: there's always the chance that you have the misfortune of dealing with some real (bleeps) who delight in having power over their customers. And even otherwise-tolerant ISPs might get upset if you start getting many thousands of hits per day.

Assuming you're not that unlucky, you can get by without registering a domain name: there are "dynamic DNS" organizations set up for people like you who will assign you a machine name within their domain, and let you update the DNS record as your IP address changes. I haven't done it, myself, so I can't offer any recommendations about which to use. But a google search for "dynamic DNS" should get you some links to praise and horror stories from users, and to practical how-to information.

I would not make that decision just based on speculation. If your server is used for private non commercial purposes or use excessive bandwidth (ISPs usually limit your up/down bandwidth), then they probably won't care. There are many server based applications in use today like pc anywhere and home automation programs that are server based.

Google forum search: Use Google Advanced Search and use Http://forum.arduino.cc/index in the "site or domain:" box.

There are two issues. One is letting the outside world know your IP address. The other is routing traffic from your router to the arduino.

If you are just a residential customer, most likely you have a dynamic IP address, which means every time your router connects it has a different IP address. There are services that allow you to assign a name to your router even if you have dynamic IP. Perhaps the best known such service is DynDNS. Google it. In order for DnyDNS to work you have to send it your IP address periodically. That's simple enough that an arduino could do it, directions are here: http://www.dyndns.com/developers/specs/syntax.html

Issue 2 is routing traffic through your router. What you want is called "port forwarding." Your ISP has to allow this, and your router has to allow it. Verizon allows it for me, and they hate everything, so odds are your ISP allows it. The idea is you tell your router if it receives any traffic on a certain port it should forward it to a certain IP address on your network. You give the router the IP address of your Arduino. To connect to a specific port on the router, put it after a colon in the address bar of your router. Like this:http://www.myaddress.com:8888

Where 8888 is the port.

Normally your router is at 192.168.1.1, see if you can connect with your browser and poke around lookingn for info on port forwarding.

I have found countless instructions for getting such a thing to work, but I have an Apple Airport. And you all may be well aware how "closed-source" apple is. I'm not sure how much of this stuff I can actually do... But thanks anyway! I'll update this post if I get anything working.

The Airport is a router but it's not the one you need to mess with. How does your internet enter the building? Is it DSL, cable? Where the wire comes in there should be a box. Sometimes it's called a modem but it's a modem/router. That's where you need to set up your port forwarding.

When you initialize ethernet on your Arduino, do you have to set the default gateway? If not, it uses 192.168.1.1 by default. Point your browser to http://192.168.1.1 and see what it tells you.

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